I am working on an AES256 encryption algorithm in a Kotlin Multiplatform project targeting iOS.
I checked some of the existing libraries that implement this in pure Kotlin (such as krypto), but none of them fit the requirements that I have from the rest of the code (which already has implementations in JVM and JS, so cannot be changed).
Coming from an iOS background, I decided to use CommonCrypto. I adapted and ported the code from here but I am stuck on how to pass a ULong by reference to the CCCrypt function and later retrieve its value.
I read very carefully the Kotlin docs on the C Interop and the Objective-C Interop, but I couldn't find any example that would explain how to deal with my case.
In particular, my issue is with the numBytesEncrypted variable (see code below). I need to pass that by reference to the CCCrypt function and then read its value to instantiate the resulting NSData with the correct length. In Objective-C/Swift, I would prefix the variable with & when calling the function.
However, Kotlin doesn't support the & operator. If I understood correctly from the docs, the replacement in Native is CValueRef (docs), so I used the cValue shorthand to get a ref of the correct type (which should be size_t, aka. ULong).
I tried to instantiate the CValueRef in two ways, both seem to be working as far as type-checking goes:
val numBytesEncrypted = cValue<ULongVar>()
// or
val numBytesEncrypted = cValue<ULongVarOf<size_t>>()
And then I used this code to get the value after the function has been executed:
numBytesEncrypted.getPointer(MemScope()).pointed.value // <- this always returns 0.
I tested the rest of the algorithm and it works perfectly (the encrypted value is the expected one and decryption also works), but without knowing the length of the encrypted/decrypted data, I cannot instantiate the NSData (and the resulting ByteArray) correctly.
Here's the full function code, including an extension I wrote (based on code found here) to convert NSData to ByteArray:
private fun process(operation: CCOperation, key: ByteArray, initVector: ByteArray, data: ByteArray): ByteArray = memScoped {
bzero(key.refTo(0), key.size.convert())
val dataLength = data.size
val buffSize = (dataLength + kCCBlockSizeAES128.toInt()).convert<size_t>()
val buff = platform.posix.malloc(buffSize)
val numBytesEncrypted = cValue<ULongVarOf<size_t>>() // <- or the other way above
val status = CCCrypt(
operation,
kCCAlgorithmAES128,
kCCOptionPKCS7Padding,
key.refTo(0), kCCKeySizeAES256.convert(),
initVector.refTo(0),
data.refTo(0), data.size.convert(),
buff, buffSize,
numBytesEncrypted
)
if (status == kCCSuccess) {
return NSData.dataWithBytesNoCopy(
buff,
numBytesEncrypted.getPointer(MemScope()).pointed.value // <- this always returns the original value of the variable or 0.
)
.toByteArray()
}
free(buff)
return byteArrayOfUnsigned(0)
}
fun NSData.toByteArray(): ByteArray = ByteArray([email protected]()).apply {
usePinned {
memcpy(it.addressOf(0), [email protected], [email protected])
}
}
Also, here's the unit test that I'm using to verify the code:
@Test
fun testAes256() {
val initVector = ByteArray(16) { _ -> 0x00 }
val key = ByteArray(32) { _ -> 0x00 }
val data = ByteArray(1) { _ -> 0x01 }
val expectedEncrypted = byteArrayOfUnsigned(0xFE, 0x2D, 0xE0, 0xEE, 0xF3, 0x2A, 0x05, 0x10, 0xDC, 0x31, 0x2E, 0xD7, 0x7D, 0x12, 0x93, 0xEB)
val encrypted = process(kCCEncrypt, key, initVector, data)
val decrypted = process(kCCDecrypt, key, initVector, encrypted)
assertArraysEquals(expectedEncrypted, encrypted)
assertArraysEquals(data, decrypted)
}
private fun assertArraysEquals(expected: ByteArray, actual: ByteArray) {
if (expected.size != actual.size) asserter.fail("size is different. Expected: ${expected.size}, actual: ${actual.size}")
(expected.indices).forEach { pos ->
if (expected[pos] != actual[pos]) asserter.fail("entry at pos $pos different \n expected: ${debug(expected)} \n actual : ${debug(actual)}")
}
}
Thank you in advance!